Control and switch design for multiple phase change loops

ABSTRACT

A cooling system includes an evaporator, connected through fluid lines to a first condenser, a second condenser, a compressor, and a thermal expansion valve. One or more valves are arranged in the fluid lines. The one or more valves operated to, in a first mode, circulate fluid between the evaporator the first condenser; in a second mode, circulate the fluid between a) the evaporator and the first condenser, and b) the evaporator, the second condenser, and the thermal expansion valve, and; in a third mode, circulate the fluid between a) the evaporator and the first condenser, and c) the evaporator, the compressor, the second condenser, and the thermal expansion valve.

TECHNICAL FIELD

Embodiments of the present disclosure relate generally to a coolingsystem for information technology (IT) equipment. More particularly,embodiments of the disclosure relate to a cooling system having multiplephase change loops.

BACKGROUND

Data centers are mission critical facilities that house a large numberof servers and IT equipment. Cooling systems provide a proper thermalenvironment for the IT equipment. It is critical to design a coolingsystem that provides cooling of IT equipment over a variety of heat loadconditions, in an efficient and durable manner.

Phase change cooling technology tends to have good heat transferperformance while keeping the temperature low. Passive phase changecooling utilizes gravity to drive the fluid with no pump needed.Controlling a passive phase change cooling system and maintaining asteady state of a passive phase change cooling loop can be a challengebecause of the inherent instability of the phase change process.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example and notlimited in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 shows a cooling system with more than one phase change loop,according to one embodiment.

FIG. 2 shows a process for controlling a cooling system, according toone embodiment.

FIG. 3 shows a switching buffer, according to one embodiment.

FIG. 4 shows a cooling system with more than one phase change loop,according to one embodiment.

FIG. 5 shows a cooling system with multiple backup condensers, accordingto one embodiment.

FIG. 6 shows an electronic shelf rack, according to one embodiment.

DETAILED DESCRIPTION

Various embodiments and aspects of the inventions will be described withreference to details discussed below, and the accompanying drawings willillustrate the various embodiments. The following description anddrawings are illustrative of the invention and are not to be construedas limiting the invention. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentinvention. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present inventions.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin conjunction with the embodiment can be included in at least oneembodiment of the invention. The appearances of the phrase “in oneembodiment” or “in one aspect” in various places in the specification donot necessarily all refer to the same embodiment or aspect.

Embodiments of the present disclosure address problems of existingcooling solutions, such as, for example, those mentioned above. In thepresent disclosure, a control and switch architecture for managing acooling system is described. The cooling system includes multiple phasechange loops that include a mix of active and passive loops.

The cooling system controls its own two-phase thermal system, and worksin an energy efficient and healthy mode (e.g., precautions are taken topreserve integrity of the system and its parts). An active phase changecooling loop is integrated with a passive phase change cooling loop withpressure control for low, medium, and high heat generation source. Thecooling system can use a switching scheme with a variation buffer on thecontrolling parameter (e.g., a pressure threshold) to ensure that thecooling system (including a compressor) works in an energy efficientway, while also improving the stability and reliability of the system.

According to one embodiment, as shown in FIG. 1 , a cooling system 100includes an evaporator 120, connected through fluid lines to a firstcondenser 102, a second condenser 104, a compressor 108, and a thermalexpansion valve 116. One or more valves 106 are arranged in the fluidlines to coordinate fluid flow from the evaporator to the condensers andthe compressor. The one or more valves are operated to, in a first mode(a ‘passive mode’), circulate fluid between the evaporator the firstcondenser. Flow to the compressor and the second condenser is restrictedin this mode.

The evaporator and the first condenser can form a thermosiphon loop thatcirculates between the evaporator and the first condenser passively,without use of a motor/pump. In such a case, the fluid in the coolingsystem is vaporized in the evaporator. The vaporized fluid flows to thefirst condenser based on natural convection—utilizing thermal expansionto create a density difference across the loop. The vaporized fluidfloats from the evaporator to the first condenser. The cooler fluid willsink from the first condenser back to the evaporator. Thus, the fluid iscirculated without a pump, while transferring thermal energy away from aheat source (e.g., IT equipment). This allows the cooling system to workpassively when less thermal transfer is required.

In a second mode (a ‘transition mode’), the one or more valves areoperated to circulate the fluid between a) the evaporator and the firstcondenser, and b) the evaporator, the second condenser, and the thermalexpansion valve. Similar to the first mode, this second mode does notuse a motor/pump. In both the first mode and the second mode, thethermal expansion valve is controlled to maintain no pressure dropacross the thermal expansion valve 116—no pressure is released in thesemodes by the thermal expansion valve.

In a third mode (an ‘extreme mode’), the one or more valves are operatedto circulate the fluid between a) the evaporator and the firstcondenser, and c) the evaporator, the compressor, the second condenser,and the thermal expansion valve. In this mode, the compressor 108 andits motor/pump is turned on by motor controller 110. The compressorgreatly increases the pressure and temperature of the fluid throughcompression. There is not a significant phase change from the inlet tothe outlet of the compressor. Due to the threshold temperature requiredto enter the third mode, all the coolant in the compressor should bevapor (gas). The second condenser then extracts significant amounts ofthis thermal energy from the fluid, converting some if not all of thevaporized fluid to liquid form. In the third mode, due to the activecompression and increased circulation of the fluid, more thermal energyis extracted at the second condenser than in the other modes. In someembodiments, the compressor is an adjustable compressor. Frequency ofthe compressor can be adjusted, e.g., by the motor control and/orcontroller, to provide varying cooling capabilities. At higherfrequencies, the cooling system is capable of extracting greater thermalenergy from a heat source.

In this third mode, the thermal expansion valve 116 is controlled toreduce the pressure and temperature of the fluid (e.g., a coolant)greatly. There is no phase change from its inlet to the outlet. All thecoolant is liquid. The pressure difference over the thermal expansionvalve can be controlled, (e.g., by a controller unit, with a controlcommand). As discussed, when the compressor is not working (e.g.,bypassed in the first and second mode), there is no pressure differencebefore and after the thermal expansion valve, by control. Whencompressor is working, the temperature and pressure of the fluid arereduced at the outlet of the thermal expansion valve. In the third mode,the controller determines how much to reduce the pressure (e.g., ‘reducepressure X amount’) and can control this reduction to varying amounts ofpressure by an analog or digital control command. The reduction commandcan be determined based on the pressure at P4.

The system includes pressure sensors 124 that are arranged in thecooling system to measure pressure in the fluid lines at locations P1,P2, P3, and P4. One or more pressure sensors inside the compressor 108sense pressure P1, thereby monitoring the working status of thecompressor—to determine whether or not the compressor is compressingfluid. Pressure sensors P3 and P2 sense pressure at the inlet and outletof the compressor, monitoring the suction pressure and dischargepressure of the compressor. P1 monitors the pressure inside thecompressor. If one or more of pressures P1, P2, or P3 is too low (e.g.,below a threshold pressure), this indicates that the compressor is notworking properly and/or there is too much liquid in the compressor.Under this condition, the control unit and/or the motor control can stopthe compressor from running, to prevent damage to the compressor and/orthe cooling system. The motor control can be implemented as acombination of hardware (e.g., programmable logic, electronic circuits,electrical switches and relays, and/or processors) and software(instructions stored in machine readable memory), A pressure sensormeasures an outlet pressure of the evaporator P4, e.g., on one side ofthe three-way valve. This pressure can be used by the cooling system tomonitor the coolant flowing out of the evaporator.

When the cooling system operates in the third mode, the fluid can bemeasured at or prior to the inlet of the evaporator, or between theevaporator and the thermal expansion valve. The measurement indicates anamount of the fluid that is in vapor form. This amount can be describedas a percentage, ratio, or volume. This measurement can be performedwith a flow meter 114 or the cooling system. The flow meter can be anultrasound flow meter that produces one or more signals that indicatehow much of the fluid is in vapor form. The vapor measurement can beperformed with other equivalent technology. Based on the measurementthat indicates an amount of the fluid that is in vapor form at an inletof the evaporator, the one or more valves are operated to increase thefluid circulation from the evaporator to the compressor, and the secondcondenser (i.e. through the extreme mode loop).

It should be understood that each of the evaporator, the condensers, andthe valves have inlets and outlets. Inlets are shown with arrows of thefluid lines pointing into the respective component, while outlets areshown with arrows of the fluid lines pointing out of the respectivecomponent. The condensers of any of the cooling system embodiments canbe liquid cooled or air cooled condensers. Fluid in the fluid lines canbe a coolant (e.g., water, glycol, etc.) that can be in liquid or vapor(gas) form.

The cooling system transitions between the first mode, the second mode,and the third mode automatically, based on pressure P4, which can bedescribed as the pressure in the fluid line at the outlet of theevaporator. A controller unit 122 can monitor the pressure P1-P4 andcontrol each of the one or more valves 106 in a coordinated manner todirect and restrict fluid circulation as described by the first, second,and third mode. The controller unit 122 can include one or moreprocessors that perform programmed instructions stored onmachine-readable medium. Additionally, or alternatively, the controllerunit can include electronic hardware logic such as field programmablegate arrays (FPGAs), complex programmable logic devices (CPLDs), orother equivalent technologies. The controller unit can be integratedinto one or more of the valves, packaged separately as a standalonedevice, and/or a combination of multiple devices.

The one or more valves can be electro-mechanically controlled by thecontroller unit. For example, a valve can have an actuator (e.g.,solenoid and magnetically attractable members) that cause the valve toopen and close when activated and deactivated. Additionally, oralternatively, valves can include a step motor or other equivalenttechnology. The controller unit can also communicate with the motorcontrol 110 to command the motor control to turn the compressor on oroff.

Open ratios of the valves can also be controlled, e.g., proportionate toa command given by the controller unit. For example, when the flow meter114 shows that vapor at the inlet of the evaporator is above a thresholdamount, the controller can command the one or more valves to be ‘open X%’. The valve responds by opening to X %, thereby increasing fluidcirculation to the compressor (assuming that the compressor is notbypassed). When the vapor amount is reduced, the opening of the valvecan similarly be commanded to be smaller, e.g., ‘open Y %’. The one ormore valves can, in this manner, control fluid circulation to thecompressor in the third mode. The command can be proportionate to theamount of fluid in vapor form that is sensed at the inlet of theevaporator.

In some embodiments, the one or more valves includes a three-way valve112 that is operated to fluidly connect the evaporator to only the firstcondenser in the first mode. The three-way valve can be operated to, inthe first mode, restrict fluid circulation to the first loop (thepassive loop). In this first loop, fluid circulates from the evaporatorto the first condenser, and then back to the evaporator.

In the second mode the three-way valve is operated to still circulatefluid to the first loop, but also allows fluid flow through the secondloop (the transition loop). In this loop, the fluid circulates from theevaporator to the bypass valve to the second compressor to the thermalexpansion valve and back to the evaporator. As such, in this mode, thebypass valve is operated to be open.

In the third mode, the three-way valve is operated to allow fluid flowin the first loop and also to the compressor 108. The one or more valvescan include a bypass valve 118 that is fluidly connected in parallelwith the compressor and in series with the second condenser. In thethird mode, the bypass valve can be operated to close, which forcesfluid circulation through the third loop (the extreme loop). In thisloop fluid circulates from the evaporator to the compressor to thesecond condenser to the thermal expansion valve and back to theevaporator. In all three modes, fluid flows through the first loop.

In such a manner as described with respect to FIG. 1 , the coolingsystem has an active phase change cooling (the extreme loop) integratedwith passive phase change cooling (the passive loop and transitionloop). Circulation in these loops is controlled through pressure, toaccommodate low and high heat generation sources, under differentthermal conditions.

FIG. 2 shows a process 200 for controlling the embodiments of thecooling system described in this disclosure, such as those shown inFIGS. 1, 4 and 5 . This process can be performed by a controller unit ofthe cooling system. The entire system is controlled to work in threemodes. The cooling system transitions between the first mode, the secondmode, and the third mode automatically, based on pressure at an outletof the evaporator (shown in FIG. 1 as P4).

In the first mode (passive mode), only the evaporator and the firstcondenser are active (the passive loop). The cooling system, atoperation 202, transitions into the first mode when the pressure at theoutlet of the evaporator (P₄) is below a first pressure thresholdP_(th1). Based on that condition, the process continues to operation 204where the cooling system runs passive mode. One or more valves of thecooling system (e.g., a three-way valve) are adjusted to circulate fluidthrough only the evaporator and the first condenser. No pressuredifference is present over the thermal expansion valve. The vapor iscooled through the condenser without going through the compressor northe second condenser of the cooling system. As described, the force todrive fluid moving is gravity and buoyancy force. The physical mechanismis a thermosiphon.

In the transition mode, the vapor is cooled through both the first andsecond condensers but the compressor is by-passed and left off. Thesecond condenser operates to supplement the first condenser, in apassive manner. At operation 206, the cooling system transitions intothe second mode when the pressure at the outlet of the evaporator isabove the first pressure threshold P_(th1) and below a second pressurethreshold P_(th2). Based on that condition, at process continues tooperation 208 where the cooling system runs in transition mode. The oneor more valves are adjusted to circulate fluid in both a) the evaporatorand first condenser (passive loop), and b) the evaporator and secondcondenser (transition loop). The transition loop also includes thethermal expansion valve, but in the transition mode, the thermalexpansion valve does not release pressure.

In the extreme mode, the fluid, which is mostly or completely in vaporform, is cooled partially by the condenser and partially through thevapor compression loop (the extreme loop) that includes the evaporator,the compressor, the second condenser, and the thermal expansion valve.At operation 210, the cooling system transitions into the third modewhen the pressure at the outlet of the evaporator (P4) is above thesecond pressure threshold P_(th2). Based on that condition, the processcontinues to operation 212 where the cooling system runs in extrememode. The one or more valves are adjusted, e.g., by closing the bypassvalve while keeping maintaining fluid connectivity between theevaporator and the compressor, to coordinate fluid circulation betweena) the evaporator and the first condenser (the passive loop), and c) theevaporator, compressor, and second condenser (the extreme loop).Pressure drops across the thermal expansion valve. This third loop formsa chiller, thereby extracting large quantities of thermal energy fromthe heat source when required.

When in the extreme mode, operation 216 can check whether there isexcessive vapor entering the inlet of the evaporator. As discussed inrelation to FIG. 1 , a flow meter can take measurements used todetermine how much of the fluid at the inlet of the evaporator is invapor form. If the vapor is greater than a threshold amount, e.g., >10%,the process can proceed to operation 214 to adjust the one or morevalves (e.g., the three-way valve) to direct more vapor to thecompressor. This will further increase the total thermal transfer of thesystem, extracting more thermal energy from the fluid at the secondcondenser, resulting in less vapor at the inlet of the evaporator.

As discussed, pressure is used for controlling the valves open status aswell as the open ratio (e.g., 50%, 75%, etc.), and the on/off state ofthe compressor. The on/off state of the compressor is limited to thethird mode (extreme mode) to ensure that only vapor enters thecompressor to protect the compressor from liquid-form fluid which candamage the compressor, and at the same time, minimize the compressor usefor improved efficiency and shelf-life. The compressor is thus operatedin a healthy manner.

When the pressure out the outlet of the evaporator (P4) is below P_(th1)(first mode), this indicates that the heat source under the evaporatorgenerates low heat so that both vapor and liquid exist before thethree-way valve and vapor can fully condense in the first condenser. Inother words, based on this pressure, the passive loop is sufficient totransfer thermal energy away from the heat source. In some embodiments,P_(th1) is 15 psi. In some embodiments, P_(th2) is 130 psi. It should beunderstood, however, that the threshold can vary based on applicationsuch as, for example, the type of fluid used and the phase changetemperature of such a fluid. Such thresholds can be determined throughtest and experimentation.

When P₄ is greater than P_(th1) but less than P_(th2) (the second mode),this indicates that heat generation of the heat source is at anintermediate level, and both the first condenser and the secondcondenser are needed to condense the vapor. Thus, the system can utilizemultiple condensers in the second mode to adjust for higher heat loads,but still remain passive.

When P₄ is greater than P_(th2) (the third mode) this indicates that theheat generation of the heat source is high. This situation includes whencoolant is completely vaporized (100%) at the outlet of the evaporator.The extreme mode becomes active under this condition and the compressorprovides additional cooling capacity.

FIG. 3 shows switching (transitioning) cut-offs are defined with buffersaround a pressure threshold to ensure the compressor operates withoutexcessive start and stop cycles. Buffers DP1 (a first predefined amountof pressure) and DP2 (a second predefined amount of pressure) can bedefined relative to P_(th2). When the pressure P4 is larger thanP_(th2), it must continue to P_(th2)+DP1 in order for the controller toconsider P4 as larger than P_(th2). As described, when this happens, thecontroller can transition the cooling system to third mode. Similarly,when the pressure is lower than P_(th2), it must continue until it isunder P_(th2)+DP2. When that happens, the controller can transition thecooling system out of the third mode, e.g., into the second mode.

It should be understood that, although FIG. 1 shows the one or morevalves 106 to include a three-way valve 112 and a bypass valve 118 tocoordinate fluid circulation in the first, second, and third modes,other arrangements of controllable valves can be implemented, as shownin FIG. 4 . For example, in FIG. 4 , a cooling system 400 includes oneor more valves 402, which can include a three valves. A valve 404controls fluid connectivity in the passive loop. Being that in all threemodes, the passive loop can remain open, in some embodiments, the valve404 is not present, and instead, the fluid line is always connected tobetween the evaporator and the condenser. Another valve 408 bypasses thecompressor to control fluid connectivity to the transition loop. Anothervalve 406 controls fluid connectivity in the extreme loop. In the secondmode, the bypass valve 408 can be open while valve 406 is closed. In thethird mode, the bypass valve 408 can be closed, and valve 406 can beopened. As discussed, the compressor valve 406 can be adjusted based onhow much vapor is flowing back to the evaporator.

Other arrangements of one or more valves can also be implemented andcoordinated to control a cooling system to operate in the modesdescribed in the present disclosure. All aspects described with respectto the other embodiments of the cooling system, e.g., those describedwith respect to FIGS. 1-3 and 5-6 , also apply to FIG. 4 , other than aparticular three-way valve arrangement.

In addition, referring to FIG. 5 , a cooling system 500 has multiplebackup condensers 502, in place of the second condenser. In other words,the second condenser includes a plurality of condensers. Thesecondensers can share a single bypass valve, as shown. All other aspectsdescribed with respect to the other embodiments of the cooling system,e.g., those described with respect to FIGS. 1-4 and 6 , also apply toFIG. 5 . Thus, in the second mode, multiple secondary condensers can beimplemented, in parallel.

As described, the cooling system as described in the embodiments above,can be thermally connected to a heat source (e.g., a GPU, CPU, or otherIT equipment). The IT equipment can be housed on an electronic rack.FIG. 6 is a block diagram illustrating an example of an electronic rackusing a cooling system 940, which can be any of the cooling systemembodiments described above. Electronic rack 900 may contain one or moreservers, each server having one or more processing units attached to abottom of any of the cooling systems (e.g., cooling system 940)described above. Referring to FIG. 6 , according to one embodiment,electronic rack 900 includes, but is not limited to, CDU 901, rackmanagement unit (RMU) 902 (optional), and one or more server blades903A-903D (collectively referred to as server blades 903). Server blades903 can be inserted into an array of server slots respectively fromfrontend 904 or backend 905 of electronic rack 900. Note that althoughthere are only five server blades 903A-903E shown here, more or fewerserver blades may be maintained within electronic rack 900. Also notethat the particular positions of CDU 901, RMU 902, and server blades 903are shown for the purpose of illustration only; other arrangements orconfigurations of CDU 901, RMU 902, and server blades 903 may also beimplemented. Note that electronic rack 900 can be either open to theenvironment or partially contained by a rack container, as long as thecooling fans can generate airflows from the frontend to the backend.

In addition, for each of the server blades 903, a fan module isassociated with the server blade. In this embodiment, fan modules931A-931E, collectively referred to as fan modules 931, and areassociated with server blades 903A-903E respectively. Each of the fanmodules 931 includes one or more cooling fans. Fan modules 931 may bemounted on the backends of server blades 903 to generate airflowsflowing from frontend 904, traveling through the air space of the severblades 903, and existing at backend 905 of electronic rack 900.

In one embodiment, CDU 901 mainly includes heat exchanger 911, liquidpump 912, and a pump controller (not shown), and some other componentssuch as a liquid reservoir, a power supply, monitoring sensors and soon. Heat exchanger 911 may be a liquid-to-liquid heat exchanger. Heatexchanger 911 includes a first loop with inlet and outlet ports having afirst pair of liquid connectors coupled to external liquid supply/returnlines 931-932 to form a primary loop. The connectors coupled to theexternal liquid supply/return lines 931-932 may be disposed or mountedon backend 905 of electronic rack 900. The liquid supply/return lines931-932 are coupled to a set of room manifolds, which are coupled to anexternal heat removal system, or extremal cooling loop. In addition,heat exchanger 911 further includes a second loop with two ports havinga second pair of liquid connectors coupled to liquid manifold 925 toform a secondary loop, which may include a supply manifold to supplycooling liquid to server blades 903 and a return manifold to returnwarmer liquid back to CDU 901. Note that CDUs 901 can be any kind ofCDUs commercially available or customized ones. Thus, the details ofCDUs 901 will not be described herein.

Each of server blades 903 may include one or more IT components (e.g.,central processing units or CPUs, graphical processing units (GPUs),memory, and/or storage devices). Each IT component may perform dataprocessing tasks, where the IT component may include software installedin a storage device, loaded into the memory, and executed by one or moreprocessors to perform the data processing tasks. At least some of theseIT components may be attached to the bottom of any of the cooling systemas described above. Server blades 903 may include a host server(referred to as a host node) coupled to one or more compute servers(also referred to as computing nodes, such as CPU server and GPUserver). The host server (having one or more CPUs) typically interfaceswith clients over a network (e.g., Internet) to receive a request for aparticular service such as storage services (e.g., cloud-based storageservices such as backup and/or restoration), executing an application toperform certain operations (e.g., image processing, deep data learningalgorithms or modeling, etc., as a part of a software-as-a-service orSaaS platform). In response to the request, the host server distributesthe tasks to one or more of the performance computing nodes or computeservers (having one or more GPUs) managed by the host server. Theperformance compute servers perform the actual tasks, which may generateheat during the operations.

Electronic rack 900 further includes optional RMU 902 configured toprovide and manage power supplied to servers 903, fan modules 931, andCDU 901. RMU 902 may be coupled to a power supply unit (not shown) tomanage the power consumption of the power supply unit. The power supplyunit may include the necessary circuitry (e.g., an alternating current(AC) to direct current (DC) or DC to DC power converter, backup battery,transformer, or regulator, etc.) to provide power to the rest of thecomponents of electronic rack 900.

In one embodiment, RMU 902 includes optimization module 921 and rackmanagement controller (RMC) 922. RMC 922 may include a monitor tomonitor operating status of various components within electronic rack900, such as, for example, computing nodes 903, CDU 901, and fan modules931. Specifically, the monitor receives operating data from varioussensors representing the operating environments of electronic rack 900.For example, the monitor may receive operating data representingtemperatures of the processors, cooling liquid, and airflows, which maybe captured and collected via various temperature sensors. The monitormay also receive data representing the fan power and pump powergenerated by the fan modules 931 and liquid pump 912, which may beproportional to their respective speeds. These operating data arereferred to as real-time operating data. Note that the monitor may beimplemented as a separate module within RMU 902.

Based on the operating data, optimization module 921 performs anoptimization using a predetermined optimization function or optimizationmodel to derive a set of optimal fan speeds for fan modules 931 and anoptimal pump speed for liquid pump 912, such that the total powerconsumption of liquid pump 912 and fan modules 931 reaches minimum,while the operating data associated with liquid pump 912 and coolingfans of fan modules 931 are within their respective designedspecifications. Once the optimal pump speed and optimal fan speeds havebeen determined, RMC 922 configures liquid pump 912 and cooling fans offan modules 931 based on the optimal pump speed and fan speeds.

As an example, based on the optimal pump speed, RMC 922 communicateswith a pump controller of CDU 901 to control the speed of liquid pump912, which in turn controls a liquid flow rate of cooling liquidsupplied to the liquid manifold 925 to be distributed to at least someof server blades 903. Therefore, the operating condition and thecorresponding cooling device performance is adjusted. Similarly, basedon the optimal fan speeds, RMC 922 communicates with each of the fanmodules 931 to control the speed of each cooling fan of the fan modules931, which in turn control the airflow rates of the fan modules 931.Note that each of fan modules 931 may be individually controlled withits specific optimal fan speed, and different fan modules and/ordifferent cooling fans within the same fan module may have differentoptimal fan speeds.

Note that some or all of the IT components of servers 903 may beattached to any one of the cooling systems described above, e.g., at theevaporator of the cooling system. One server may utilize air coolingwhile another server may utilize liquid cooling. Alternatively, one ITcomponent of a server may utilize air cooling while another IT componentof the same server may utilize liquid cooling.

It should be understood that the various features shown with respect toone figure can also be present in other embodiments of differentfeature.

In the foregoing specification, embodiments of the invention have beendescribed with reference to specific exemplary embodiments thereof. Itwill be evident that various modifications may be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the following claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense.

What is claimed is:
 1. A cooling system, comprising: an evaporator,connected through fluid lines to a first condenser, a second condenser,a compressor, and a thermal expansion valve; and one or more valves thatare arranged in the fluid lines, the one or more valves operated to in afirst mode, circulate fluid between the evaporator the first condenser,in a second mode, circulate the fluid between a) the evaporator and thefirst condenser, and b) the evaporator, the second condenser, and thethermal expansion valve, and in a third mode, circulate the fluidbetween a) the evaporator and the first condenser, and c) theevaporator, the compressor, the second condenser, and the thermalexpansion valve.
 2. The cooling system of claim 1, wherein the coolingsystem transitions between the first mode, the second mode, and thethird mode automatically, based on pressure at an outlet of theevaporator.
 3. The cooling system of claim 2, wherein the cooling systemtransitions into the first mode when the pressure at the outlet of theevaporator is below a first pressure threshold.
 4. The cooling system ofclaim 3, wherein the cooling system transitions into the second modewhen the pressure at the outlet of the evaporator is above the firstpressure threshold and below a second pressure threshold.
 5. The coolingsystem of claim 4, wherein the cooling system transitions into the thirdmode when the pressure at the outlet of the evaporator is above thesecond pressure threshold.
 6. The cooling system of claim 4, wherein thecooling system transitions into the third mode when the pressure at theoutlet of the evaporator is greater than the second pressure thresholdby a first predefined amount, and transitions out of the third mode whenthe pressure at the outlet of the evaporator is less than the secondpressure threshold by a second predefined amount.
 7. The cooling systemof claim 1, wherein the evaporator and the first condenser form athermosiphon loop that circulates between the evaporator and the firstcondenser passively, without use of a motor.
 8. The cooling system ofclaim 1, wherein the compressor is turned on in the third mode andturned off in the first mode and the second mode.
 9. The cooling systemof claim 1, wherein, in the third mode, pressure is released through thethermal expansion valve creating a pressure drop across the thermalexpansion valve, and in the second mode, the pressure across the thermalexpansion valve is maintained to be uniform.
 10. The cooling system ofclaim 1, wherein, in the third mode, based on a measurement thatindicates an amount of the fluid that is in vapor form at an inlet ofthe evaporator, the one or more valves are operated to increase fluidcirculation from the evaporator to the compressor, and the secondcondenser.
 11. The cooling system of claim 1, wherein the one or morevalves includes a three-way valve that is operated to fluidly connectthe evaporator to only the first condenser in the first mode.
 12. Thecooling system of claim 11, wherein the one or more valves includes abypass valve connected in parallel with the compressor and in serieswith the second condenser that is operated to in the second mode, openand bypass the compressor resulting in fluid circulation from theevaporator to the second condenser, and in the third mode, close,resulting in fluid circulation from through the compressor to the secondcondenser, in the third mode.
 13. The cooling system of claim 1, whereinthe second condenser consists of a plurality of secondary condensers.14. The cooling system of claim 1, wherein the compressor is anadjustable compressor, and adjustments of the compressor provide varyingcooling capabilities.
 15. An electronics rack that houses IT equipmentand a cooling system that is thermally coupled to the IT equipment, thecooling system including an evaporator, connected through fluid lines toa first condenser, a second condenser, a compressor, and a thermalexpansion valve; and one or more valves that are arranged in the fluidlines, the one or more valves operated to in a first mode, circulatefluid between the evaporator the first condenser, in a second mode,circulate the fluid between a) the evaporator and the first condenser,and b) the evaporator, the second condenser, and the thermal expansionvalve, and in a third mode, circulate the fluid between a) theevaporator and the first condenser, and c) the evaporator, thecompressor, the second condenser, and the thermal expansion valve. 16.The electronics rack of claim 15, wherein the cooling system transitionsbetween the first mode, the second mode, and the third modeautomatically, based on pressure at an outlet of the evaporator.
 17. Theelectronics rack of claim 16, wherein the cooling system transitionsinto the first mode when the pressure at the outlet of the evaporator isbelow a first pressure threshold.
 18. The electronics rack of claim 17,wherein the cooling system transitions into the second mode when thepressure at the outlet of the evaporator is above the first pressurethreshold and below a second pressure threshold.
 19. The electronicsrack of claim 18, wherein the cooling system transitions into the thirdmode when the pressure at the outlet of the evaporator is above thesecond pressure threshold.
 20. The electronics rack of claim 15, whereinthe evaporator and the first condenser form a thermosiphon loop thatcirculates between the evaporator and the first condenser passively,without use of a motor.